It is known that photographic images having very high contrast can be formed using certain silver halides, and a method for forming such photographic images is generally practiced in the field of photomechanical image forming processes.
For example, a method of obtaining line originals or dot images having high contrast and a high optical density where an image portion and a non-image portion are clearly distinguished by treating a lith type silver halide photographic material comprising a silver chlorobromide having a silver chrolide content of at least 50 mol % with a hydroquinone developing solution wherein the effective concentration of sulfite ions is relatively low (generally, up to about 0.1 mol/liter), is known. However, in this type of method, because the concentration of sulfite ions is low, the developer is unstable due to aerial oxidation, and thus, workers in this have made various attempts over the years to stabilize the developer solution.
Therefore, an image forming system has been desired wherein the instability of the image formation due to such a developing method (a lith developing system) is obviated, and development can be effected using a processing solution having a good shelf stability to provide resulting images with superhigh contrast, particularly, having a gamma of about 10 or higher. As described in U.S. Pat. Nos. 4,166,742, 4,168,977, 4,221,857, 4,224,401, 4,243,739, 4,272,606 and 4,311,781, a system has been suggested wherein a surface latent image type (negative type) silver halide photographic material, to which a specific acylhydrazine compound has been incorporated, is treated at a pH of 11.0 to 12.3 with a developing solution containing not less than 0.15 mol of a sulfite preservative per liter and having a good shelf stability to form superhigh contrast negative images having a gamma of higher than 10. Further, this type of image forming system allows both silver iodobromide and silver chloroiodobromide to be used as the silver halide, whereas in prior superhigh contrast image forming methods, only silver chlorobromide having a high content of silver chloride could be employed.
However, in this type of image forming method, when a large amount of film high in optical density is processed, the pH of the processing solution is lowered or the concentration of Br.sup..crclbar. ions increases, resulting in an undesirable effects on the resulting images, i.e., a decrease in sensitivity and gamma. Although it has been reported that such problems can be lessened by using a hydrazine compound having a ureido group as disclosed in Japanese Patent Application (OPI) No. 67843/81 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") or using an onium salt or an amine compound as disclosed in Japanese Patent Application (OPI) Nos. 140340/85 and 9347/85, in these methods, because of the very high infectious developability, when a low contrast document, particularly one with fine lines such as Ming-cho type letters, is photographed, even parts that should become white fine parts become blackened, resulting in an illegible image. Such a problem also occurs when photographing a dot image, and thus, white parts among dots are liable to become blackened and the halftone gradation disadvantageously becomes very short.
Further, these methods provide remarkably high sensitivity and high contrast, but at the same time give rise to undesirable "black pepper" due to such infectious development, which is a serious problem in photomechanical processes. The term "black pepper" means black spots composed of fine developed silver grains formed at sections that have not been exposed and therefore are not image sections. In general, black peppers are frequently formed due to a decrease in sulfite ions used in a developing solution as a preservative or by a rise in the pH value of the developer, and greatly lowers the commercial value of the photosensitive material for photomechanical processes. Therefore, intensive efforst have been made to find a solution for this problem, i.e., decreasing the amount of black peppers, but a decrease in the amount of black peppers often also results in a concomitant decrease in sensitivity and gamma. Hitherto, superior methods for decreasing black peppers without interfering with high sensitivity and superhigh contrast effects obtainable by the incorporation of hydrazine derivatives are not known. However, it has been found that although certain sensitizing dyes can prevent black peppers from being formed without decreasing the sensitivity and the gamma since they have substantial absorption maximums in the visible region, undesirable remaining color then becomes a problem when the sensitizing dye remains in the film after the photographic material has been developed, fixed, and washed with water.